The triumphal arch through which I march With hurricane, fire, and snow, When the Powers of the air are chained to my chair, Is the million-colored bow; The sphere-fire above its soft colors wove, While the moist Earth was laughing below.

The "triumphal arch" is the bend of the rainbow. The "million-colored bow" is the rainbow itself. The "soft colors wove" refers to the relationship between the rainbow and the "laughing" planet below.